1. Field of the Invention
The present invention relates to improvements in boilers for electric utility or industrial use, furnaces for chemical industry, and the like which make use of pulverized solid fuel.
2. Description of the Prior Art
At first, one example of a boiler furnace in the prior art which makes use of pulverized coal as fuel, will be described with reference to FIG. 6 showing a vertical cross-section view, and FIG. 7 showing a horizontal cross-section view taken along line VII--VII in FIG. 6. In these figures, reference numeral 01 designates a furnace main body, numeral 02 designates burner main bodies, numeral 03 designates fuel nozzles, numeral 04 designates air nozzles for a main burner, numeral 05 designates pulverized coal transport pipes, numeral 06 designates combustion air lines, numeral 07 designates a coal pulverizer, numeral 08 designates a blower, numeral 09 designates pulverized coal-air mixture, numeral 10 designates combustion air, numeral 11 designates coal, numeral 12 designates conveying air, numeral 13 designates a furnace inner space or inner chamber, numeral 14 designates pulverized coal flames, numeral 15 designates main burner air lines, numeral 16 designates additional air lines, numeral 17 designates air for main burners, numeral 18 designates additional air, and numeral 19 designates additional air nozzles.
The above-described furnace main body 01 is formed in a square barrel-shape having a vertical axis, and as shown in FIG. 7, it is provided with burner main bodies 02 at corner portions in a horizontal cross-section of a furnace wall. Each burner main body 02 is provided with a plurality of (three in the illustrated example) assemblies each consisting of a fuel nozzle 03 and air nozzles 04 assembled above and below the fuel nozzle 03, as aligned vertically, and these fuel nozzles 03 and air nozzles 04 are all directed horizontally towards the inner space of the furnace.
Coal 11 fed to a coal pulverizer 07 is finely pulverized and mixed with conveying air (hot air) 12 which is fed simultaneously, to form pulverized coal-air mixture 09, and then the mixture sent to the burner main body 02 through pulverized coal transport pipes 05. The pulverized coal-air mixture sent to the burner main body 02 is injected to the furnace inner space 13 via the fuel nozzles 03. On the other hand, combustion air 10 is fed through combustion air lines 06 by a blower 08, then it is branched into main burner air 17 and additional air 18, and they are respectively injected to the furnace inner space 13 through air nozzles 04 provided in the burner main bodies 02 and through additional air nozzles 19 provided above the burner main bodies 02.
The pulverized coal-air mixture 09 injected to the furnace inner space 03 is ignited by an ignition source not shown, and burns while forming pulverized coal flames 13. In the pulverized coal flames 14, the pulverized coal burns, in the proximity of an ignition point, as reacting with oxygen supplied by the conveying air 12 forming the pulverized coal-air mixture 09 together with the pulverized coal as well as a part (in the proximity of the ignition point) of the main burner air 17, and thereafter in a main combustion zone, combustion is continued by oxygen in the remainder of the main burner air 17.
In a heretofore known boiler, since a total amount of the conveying air 12 and the main burner air 17 is made less than an amount corresponding to a stoichiometric ratio with respect to the pulverized coal injected through the fuel nozzles 03 for the purpose of suppressing production of nitrogen oxides (hereinafter abbreviated as NO.sub.x) the furnace inner space 13 from the portion of the burner main bodies 02 up to the additional air nozzles 19 is held under a reducing atmosphere condition. Accordingly, the combustion gas produced by combustion of the pulverized coal-air mixture 09 would rise through the furnace inner space 13 initially in an incomplete combustion state, and the combustion is completed by the additional air 18 injected through the additional air nozzles 19.
Also, in the heretofore known boiler, a mixing ratio of conveying air to pulverized coal in the pulverized coal-air mixture 09 was mostly chosen in the range of 2:1 to 4:1 in weight proportion generally in view of practical use of the coal pulverizer 07. That is, the pulverized coal-air mixture 09 was subjected to combustion at a mixing ratio of [conveying air]/[pulverized coal] (hereinafter abbreviated as A/C)=2-4.
Now let us consider the problems involved in the heretofore known boilers.
[1] Generally, ignitability of the pulverized coal flames 14 is improved when the following conditions are fulfilled.
1) An amount of volatile constituent in pulverized coal is high, and a fuel ratio (fixed carbon/volatile constituent) is low; PA1 2) A heat flow flux reaching a burner opening is large; PA1 3) An A/C ratio of the pulverized coal-air mixture 09 is close to 1; and PA1 4) An injection speed of the pulverized coal-air mixture 09 is slow. Accordingly, boilers satisfying the above-mentioned conditions as much as possible are considered to be favorable.
FIG. 8 is a diagram showing one example of results of practical measurements of the distribution of a heat flow flux coming from a furnace inner space 13 and reaching a furnace wall with respect to a real boiler, and FIG. 9 is a diagram showing one example of results of experiments conducted in connection with the relations between a flame propagation speed of pulverized coal and an A/C ratio of pulverized coal-air mixture. According to these diagrams, a heat flow flux coming from a furnace inner space 13 and reaching the furnace wall becomes maximum at the central portion of the furnace wall, and a flame propagation speed of pulverized coal becomes maximum at A/C.apprxeq.1 of the pulverized coal-air mixture.
Since coal having a low amount of volatile constituent and a high fuel ratio does not fulfil the condition 1) above, it is desirable to fulfil the other conditions 2), 3) and 4). However, in the heretofore known boiler, since the burner main bodies 02 were provided at the respective corner portions of the furnace main body 01 as shown in FIG. 7, a heat flow flux reaching the burner portion was small as shown in FIG. 8. On the other hand, in the case of employing coal having poor ignitability due to a low volatile constituent content, it is necessary to improve the ignitability by making the A/C ratio of the pulverized coal-air mixture 09 fed to the burner main body 02 close to 1 (See FIG. 9), but in the heretofore known boiler the A/C ratio was generally 2 to 4 due to restriction in practical use of the coal pulverizer 07, and it could not be made close to 1. In addition, although the pulverized coal-air mixture 09 becomes ready to be ignited as its injection speed is slowed down in view of the relation to a flame propagation speed, as it is injected horizontally in the case of the boiler in the prior art, if the injection speed is too slow, pulverized coal in the pulverized coal-air mixture 09 would hang down and would accumulate at the fuel nozzle 03. Therefore, the injection speed cannot be made lower than a predetermined speed.
As described above, the boilers in the prior art had a shortcoming in that it was difficult to ignite coal having a low volatile constituent content or a high fuel ratio. [2] With regard to combustion in a boiler, it is a well-known fact that an amount of production of NO.sub.x is in an inversely proportional relation to an amount of charging of additional air 18. However, in the heretofore known boiler system, since there is a problem with ignitability in the case of coal having a low volatile constituent content or a high fuel ratio, the amount of charging of the additional air 18 cannot be made large, therefore, there was a limit as to the reduction of NO.sub.x.